Power Line Communication (PLC), also known as Broadband over Power Lines (BPL), relates to communication over power lines that supply commercial power. Japanese Patent Laid-Open Publication 1995-245576 discloses that PLC usually utilizes a parallel cable of a pair of wires. In a home network utilizing power line communication, a pair of communication apparatuses perform communication between a pair of modems. Each of the communication apparatuses connects to an electrical outlet through the modem and the parallel cable.
In a single-phase three-wire system of a home, a 100VAC power system and a 200VAC power system are different in respect of a method for connecting to power lines. Specifically, a 100VAC electric appliance connects to a set of a neutral line and a voltage line. A 200VAC electric appliance connects to a pair of voltage lines. In some cases, the 200VAC electric appliance further connects to a neutral line as a ground line.
Hereinafter, a pair of voltage lines may be referred to as “a first voltage line” and “a second voltage line”. The 100VAC electric appliance connects to both the neutral line and each of the first voltage line and the second voltage line. Accordingly, the 100VAC power system has both the 100VAC electric appliance connecting the first voltage line and the 100VAC electric appliance connecting the second voltage line.
Referring to FIG. 21A, the illustrated power line communication system has three power line communication modems A, B, and C. The power line communication system performs power line communication between power line communication modems.
PLC modems A and B connect to both second voltage line P2 and neutral line P0, directly or through a 100VAC outlet (not shown in the drawing). PLC modem C connects to both first voltage line P1 and second voltage line P2, directly or through a 200VAC outlet (not shown in the drawing).
When the power line system performs power line communication utilizing the single-phase three-wire system, the method for connecting to the power line (outlet) can affect communication quality. Specifically, in a case in which PLC modems A and C perform power line communication, neutral line P0 of PLC modem A and first voltage line P1 of PLC modem C are not electrically connected as shown in FIG. 21A. Consequently, a signal travels through inductance and capacitance generated between neutral line P0 and first voltage line P1. As a result, the inductance and capacitance can attenuate the signal.
FIG. 21B shows an example of throughput (effective transmission rate) of transmission performed by PLC modems A, B, and C. A distance between PLC modems A and B is long. However, the phase of PLC modems A and B is the same; that is, PLC modem A electrically connects to PLC modem B with both neutral line P0 and first voltage line P1. Therefore, since attenuation is low, e.g., −10 dB, throughput is 50 Mbps.
On the other hand, a distance between PLC modems B and C is short. However, the phase of PLC modems B and C is different; that is, neutral line P0 does not electrically connect to first voltage line P1. Therefore, since attenuation is high, e.g., −20 dB, throughput is 30 Mbps.
Also, a distance between PLC modems C and A is long, and the phase of PLC modems C and A is different. Therefore, since attenuation is higher, e.g., −32 dB, throughput is 10 Mbps.
In the above-mentioned related art of FIG. 21B, throughput of both upstream and downstream is the same. However, throughput can be made different by an internal or external condition of the PLC modems. Influence caused by a noise source (e.g., an electric light, a dryer, and a vacuum cleaner) depends on both a position of an electrical outlet to which PLC modem connects and a path through which PLC modems perform the communication. Therefore, the position and the path can considerably change throughput.
As described above, in a case m which communication is performed utilizing power lines, the power line communication system is required to take into account conditions where the position and the path can change throughput. Japanese Patent Laid-Open Publication 2005-3332233 discloses a technical method for improving transmission status when the power line communication is performed utilizing the single-phase three-wire system.